Unit fuel injectors operated by cams, have long been used in compression ignition internal combustion engines for their accuracy and reliability. The unit injector, whether of the open or closed nozzle type, typically includes an injector body having injector orifices at one end and a cam driven injector plunger assembly mounted for reciprocal movement within the injector body. In a typical unit injector, fuel is metered into an injection chamber with the amount of fuel being controlled on a cycle by cycle basis.
As the need for higher engine efficiency and emissions abatement have increased, it has become increasingly evident that the timing of the injection event, with respect to the movement of the corresponding engine piston, must be precisely and reliably controlled during each cycle, independently from injection metering, in response to changing engine operating conditions. U.S. Pat. No. 4,721,247 discloses an open nozzle fuel injector capable of controlling the timing of injection. The injector includes an upper plunger, a lower or metering plunger and an intermediate or timing plunger. The lower plunger is biased against the timing plunger by a return spring positioned in a chamber formed between the timing plunger and the metering chamber. The spring chamber is continuously connected to drain via drain ports formed in the injector body. A variable volume hydraulic timing chamber is formed between the upper plunger and the timing plunger. Timing fluid is delivered to the timing chamber based on the "pressure/time" principle whereby the amount of timing fluid actually metered into the timing chamber is a function of primarily the supply pressure and, secondarily, the total metering time that timing fluid flows through a port in the injector. Therefore, the timing of fuel injection is controlled by varying the timing fluid pressure.
U.S. Pat. No. 5,299,738 to Genter et al. discloses an open nozzle fuel injector assembly similar to the injector disclosed in the '247 patent except that the retraction stroke of the lower plunger is limited to reduce the quantity of gas being drawn into the injector from the combustion chamber during the retraction stroke. A stop surface is formed on the injector body for abutment by a spring-biased keeper mounted on the upper end of the lower plunger and biased by a coil spring, thereby forming a cushioned stop. During the retraction stroke, the retraction movement of the lower plunger is stopped while the timing and upper plungers continue through the retraction stroke. U.S. Pat. No. 5,275,337 discloses a similar unit injector including a second spring positioned between the lower plunger and the timing plunger to assist in the retraction of the timing plunger thereby advantageously improving start of injection variability.
The injectors discussed hereinabove are incapable of achieving the degree of precise control over the timing of injection required for optimum efficiency and emissions abatement. Each of these conventional injectors undesirably permit the start of injection to vary to an unacceptable degree from one cycle to the next.
Consequently, there is a need for a unit injector having a variable volume timing chamber which is capable of precisely controlling the amount of timing fluid delivered to the timing chamber during each timing event thereby reducing start of injection variation and improving efficiency and emissions abatement.